The supply of carbon dioxide from natural sources, by-product CO.sub.2 from ammonia manufacture and hydrogen purification, is not sufficient for the present and future industrial requirements.
The potential supply of CO.sub.2 from power plant flue gas or exhaust gas from stationary internal combustion engines could furnish the required amount providing it could be economically recovered. Flue gas normally will be at or near atmospheric pressure and contains about 6-10% CO.sub.2 and aboul 2-5% oxygen. Sulfur dioxide may be an additional contaminant if the fuel source is coal or oil instead of sweet or commercial natural gas.
Most known solvents that can recover CO.sub.2 under these conditions will undergo severe solution oxidative degradation and cause corrosion, thus rendering the process uneconomical.
The removal of carbon dioxide from flue gas was practiced in the 1950's and early 1960's by extracting into a solvent the carbon dioxide from the combustion products resulting from burning a fuel. One use of such recovered CO.sub.2 was as an inert atmosphere for large annealing furnaces.
The principal solvent used in the removal of CO.sub.2 from flue gas during the aforesaid period employed an aqueous monoethanolamine (MEA) solution in the concentration range of 5-12%. The system was operated until oxidative degradation products and corrosion became sufficiently severe as to warrant discarding the solution. The plant was then cleaned and recharged with fresh solution.
Some processes were able to extend the in-service time by operating the dilute MEA solution and passing a small portion of the solution to a side stream reclamation still to remove the contaminants. Such still did remove some of the oxidative degradation products as a bottom product while taking substantially the MEA and water as an overhead product for recycle. The side stream still operated on a 2-3% side stream. This approaoh was not particularly successful because the degradation products were removed only to a limited extent in the side stream reclaimer. In addition, degradation products continued to be produced at a higher rate than normally found in the process due to the higher temperatures (temperatures above those encountered in the regenerator) necessary for operating the reclaimer still. Removal of the degradation or reaction products also favors shifting the reaction equilibrium for increased reaction rate.
Another mode of operation of the dilute solution process was the utilization of a 5-8% aqueous MEA solution with a 4-8% concentration of sodium carbonate. Sodium carbonate neutralized degradation products that were acidic in nature (formic and oxalic acid are the prime oxidation products in this environment). This mode of operation was somewhat successful but like the other two mentioned systems, was unpredictable in the length of time the system would operate before losing capacity to recover CO.sub.2.
All the processes mentioned above were extremely energy intensive due to the extremely high circulation rates necessitated by the low concentration of MEA and the very low loadings of CO.sub.2 that were considered necessary in order to minimize corrosion.
Another process described for the recovery of CO.sub.2 from a flue gas used a second combustion zone to lower residual oxygen and is described in U.S. Pat. No. 4,364,915 dated Dec. 21, 1982.
A further technique utilized copper salts as an inhibitor and is disclosed in U.S. Pat. No. 2,377,966, dated June 12, 1945. This method was used in the above mentioned systems that did not include the use of a reclaimer in the operation. Copper was only moderately successful as a corrosion inhibitor even at the low CO.sub.2 loadings and low concentrations of alkanolamine. Precipitation of elmental copper was a serious limitalion of this process and resulted in enhanced corrosion due to galvanic attack in the peripheral area of the deposited copper metal. This syslem was operated much the same way that the uninhibited aqueous MEA solution first mentioned was utilized, in that when the system became sufficiently degraded the entire solution was dumped, the internals of the plant cleaned, fresh alkanolamine charged baok to the system, and the system put back in service. The length of time the system remained on stream was again unpredictable.
The use of activated carbon or ion exchange resin to remove contaminates from aqueous alkanolamine solutions is known from U.S. Pat. Nos. 1,944,122; 2,797,188; 3,568,405; and 4,287,161. However, these patents do not suggest the surprising results obtained using an effective amount of copper salts in the akanolamine solution in conjunction with the use of activated carbon or ion exchange resin as disclosed in copending application Ser. No. 471,626, filed Mar. 3, 1983 entitled IMPROVED PROCESS FOR THE RECOVERY OF CO.sub.2 FROM FLUE GASES by Roscoe Lamont Pearce, Charles Richard Pauley and Richard Alan Woloolt now U.S. Pat. No. 4,477,419.
In accordance with the invention there described, gas containing carbon dioxide and oxygen is contacted in the conventional manner in a suitable gas-liquid contactor with an alkanolamine solution. All of these processes employ an alkanolamine solution containing an amount of copper effective to inhibit corrosion. The actual amount of copper used can be any amount of copper greater than about 5 parts of copper per million parts of solution into which the carbon dioxide and, if present, sulfur containing acid gases (e.g. SO.sub.2 with trace amounts of other sulfur compounds, H.sub.2 S, COS, and the like) are absorbed.
As in any conventional liquid gas absorbent process the effluent (rich solvent) from the contactor is withdrawn from the bottom of the contactor and cross exchanged with solvent (lean solvent) which has been heated to release the absorbed acid gases, in this case to produce a carbon dioxide and sulfur lean solvent. The rich solvent after heat exchange with the lean solvent is delivered to a stripper or regenerator wherein the rich solvent is contacted with rising vapors from the lower end of the stripper. The liquid in the lower end of the stripper is circulated through a reboiler wherein conventionally it is heated to about 240.degree. to 260.degree. F. (115.degree. to 126.5.degree. C.) and returned to the lower portion of the stripper or reboiler surge tank. A portion of the bottoms (now lean solvent) drawn off the stripper or reboiler surge tank is then returned to the absorption column.
The latter identified application described an invention wherein all or a portion of the alkanolamine solution at any temperature, typically passing a cool rich or cool lean solution (convenienty the lean solution after heat exchange with the rich solution from the contactor), sequentially into and through a mechanical filter, a bed of activated carbon, and, a second mechanical filter. Following this treatment, the filtered/carbon treated solution can be passed through an ion exchange resin bed, thence to the top of the contactor.
The above procedure surprisingly effectively removes ionic iron and solvent degradation products. This allows sufficient ionic copper in solution to abate corrosion, minimizes the formation of degradation products, and maintains substantially the efficiency of the alkanolamine solution.
It is to be understood that while the above preferred mode of operation includes the activated carbon treatment, mechanical filtration and ion exchange treatment, some improvement, e.g. lower corrosivity and/or degradative quality of solvent, can be achieved if only one of the unit operations is employed in treating the solvent. Thus, under certain operating conditions, activated carbon treatment can remove certain of the degradation products both by adsorption and/or absorption and its inherent filtering effects mechanically removes some of the particulate material to obtain an improvement when compared to the earlier described processes. It however has been found advantageous to couple mechanical filtration both before and after activated carbon treatment to extend the life of the carbon bed and collect the insoluble iron. Ion exchange treatment may also be employed to remove some of the degradation products, with or without either mechanical filtrations or activated carbon treatment, but the bed must be cleaned more often to avoid plugging with insoluble iron or other solid degradation products when filtration is not employed at least ahead of the exchanger. Here again, mechanical filtration is preferred to keep at a low level the insoluble iron and/or solid degradation products from plugging the bed. Likewise, the use of one or both filtration mediums as the only treatment will improve the operation of the process but not to the same degree as operating on the three unit operations, i.e. mechanical filtration, activated carbon treaLment, and ion exchange.
It has now been discovered that the copper inhibitor lost to plating out and/or when the solution is periodically or continuously, in small aliquots, reclaimed to remove from the solution those degradation and solid products which do form even though at a reduced rate when the process of the copending application is practiced, may be recovered in part and the corrosion (galvanic or stress) can be reduced by practicing the innovations of the present invention.